Recently, in the field of servers and high-end computers, there has been a dramatic increase in transmission capacity of an I/O function of communication between a central processing unit (CPU) and an external interface, due to enhanced performance by multiple CPUs. On the other hand, in light of problems of crosstalk and wiring density with a conventional electrical high-speed transmission, a study is under way of an optical interconnect technology of arranging an optoelectric conversion element and using an optical signal for a high-speed I/O.
For the optical interconnect technology, a small optical module is in demand that is several times as small as the optical module for the conventional backbone optical communication and that can be fabricated at a low cost. With respect to such an optical module, the one with a light emitting element and a light receiving element mounted face-down to a substrate is known. This optical module has the light emitting element and the light receiving element mounted face-down to the substrate transmitting the light and performs the optical communication with an optical waveguide formed beneath the substrate.
As to a configuration enabling production at a lower cost, the optical module is known that has the light receiving element or the light emitting element mounted face-down to flexible printed circuits (FPC) board made of a polyimide, etc., thin film (see, e.g., Japanese Laid-Open Patent Publication No. 2012-068539).
In such an optical module, for example, a lens sheet with a lens formed on a surface thereof is disposed, by way of a bonding sheet, on the lower surface of the flexible printed circuit board to which the optical elements are flip-chip-mounted and further, the optical waveguide is disposed beneath the lens sheet. For example, to suppress deterioration of characteristics due to the temperature rise of the optical elements, a heat sink is disposed on the upper side of the optical elements, in close contact with the optical elements and the flexible printed circuit board is held by pressure from the heat sink.
In the conventional technology described above, however, between the flexible printed circuit board and the lens sheet, a void is formed in which the bonding sheet does not intervene to secure a light path and the pressure from the heat sink causes the flexible printed circuit board to warp, arising in a problem that the optical elements are misaligned with respect to the lens.